The present invention relates to a device for measuring an ion current produced by an ion beam. It more particularly applies to the measurement of ion currents produced by ion beams from a mass spectrometer source.
Various devices for measuring ion currents, particularly in mass spectrometry are already known. The main difficulty which occurs in these measurements is due to the fact that the ion currents are very small and that, despite the use of amplifier chains and costly "amperemeters", it is difficult to directly measure currents, which are often below 10.sup.-16 amperes. The amplifier chains are generally direct current amplifier chains and it frequently occurs that the leakage currents or background noise have an amplitude higher than that of the current to be measured.
It is known that this disadvantage can in part be obviated by using measuring devices comprising alternating current amplifier chains. Two methods are known for using alternating current amplifier chains. It is possible to modulate the ion beam at the time of its formation. In a mass spectrometer, this modulation occurs at the output of the ionization chamber, prior to the space which is subject to a magnetic field for separating the ions from the sources. For example, this modulation can be carried out by a rotating or vibrating device, traversed by the ions from the chamber. The main disadvantage of this modulation at the source is that it affects the ion beam prior to the magnetic separation. It is also possible to modulate the current from ion collectors. This modulation has the main disadvantage of being performed on very low currents (10.sup.-15 to 10.sup.-16 A). As the chain amplifiers work in closed loop using negative feedbacks in which are involved very high value resistors (10.sup.10 to 10.sup.12 ohms), the inherent noise of the resistors added to the input noise of the amplifiers, their leakage current and their drift, lead to a signal-to-noise ratio, which limits the detection to 100 .mu.V at the terminals of a 10.sup.11 ohm resistor.